1. Technical Field of the Invention
The present invention relates in general to optical spectroscopy and interferometry, and in particular to the use of Micro Electro-Mechanical System (MEMS) technology in optical spectrometers.
2. Description of Related Art
Micro Electro-Mechanical Systems (MEMS) refers to the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate through microfabrication technology. For example, the microelectronics are typically fabricated using an integrated circuit (IC) process, while the micromechanical components are fabricated using compatible micromachining processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical components. MEMS devices are attractive candidates for use in spectroscopy, profilometry, environmental sensing, refractive index measurements (or material recognition), as well as several other sensor applications, due to their low cost, batch processing ability and compatibility with standard microelectronics. In addition, the small size of MEMS devices facilitates the integration of such MEMS devices into mobile and hand held devices.
Moreover, MEMS technology, with its numerous actuation techniques, enables the realization of new functions and features of photonic devices, such as optical tunability and dynamic sensing applications. For example, by using MEMS actuation (electrostatic, magnetic or thermal) to control a movable mirror of a Michelson Interferometer, small displacements in the interferometer optical path length can be introduced, and consequently, a differential phase between the interfering beams can be obtained. The resulting differential phase can be used to measure the spectral response of the interferometer beam (e.g., using Fourier Transform Spectroscopy), the velocity of the moving mirror (e.g., using the Doppler Effect), or simply as an optical phase delay element.
In MEMS-based spectrometers, beam splitting is typically performed using a thin wall of silicon (Si) or glass. For example, the beam splitter could be a silicon wall or simply an air-silicon (or any other material) interface. Such structures have the advantage of complete integration, since the mirrors and beam splitters are all fabricated in a single, self-aligned lithography step, without the need for any additional assembly of extra elements. However, in such structures, the optical beams typically pass through silicon in one arm, while the second arm is free from silicon (i.e., propagation in air only). As the silicon (or any other equivalent material for the beam splitter) has a refractive index that varies with the wavelength, dispersion may result due to the introduction of a phase error in the interferometer (i.e., a phase shift that is dependant on the wavelength).
To overcome this phase error, a complex Fourier transform (FT) is needed, instead of a simple cosine transform. In practice, the complex FT necessitates that the mirror move in the positive and negative directions with respect to its zero path difference position. Thus, for a mirror moving a distance L, the wavelength resolution is governed by only L/2 mirror displacement. However, the mirror motion in MEMS technology is usually limited by the full travel range of the actuator used to drive the mirror. Therefore, a loss in the travel range due to the phase error correction limits the wavelength resolution of the resultant spectrometer.
Another problem with existing MEMS-based spectrometers results from the fabrication process itself. Many MEMS-based spectrometers utilize a Deep Reactive Ion Etching (DRIE) on Silicon on Insulator (SOI) wafer technology to form the optical mirrors and beam splitters. Although DRIE allows the integration of different components using a simple lithographic process, DRIE suffers from poor verticality of the walls used to form the optical mirrors and beam splitters of the interferometer. For example, in DRIE technology, the state of the art of the wall verticality is on the order of 0.5 degrees with respect to the line vertical to the substrate, which is considered large in any interferometric measuring system. Typically, the accepted verticality angle (considered as a tilt angle) is on the order of few milliradians (<0.1 degree). This represents an obstacle for any DRIE based spectrometer, since the large tilt angle results in a reduction in the visibility and an increase in the insertion loss of the structure. In addition, the large tilt angle can also affect the wavelength accuracy and resolution of the interferometer. Moreover, the large tilt angle cannot be avoided by aligning the input beam, since there is an inherent misalignment between the beam splitter and the two acting mirrors.
Therefore, there is a need for a balanced architecture for the spectrometer, such that the tilt angle or verticality in the DRIE process used to create the mirrors and beam splitter walls are compensated in both arms, and the dispersion in one arm is balanced by a similar dispersion in the second arm.